The present invention relates to a flexible circuit board, a flexible circuit board mounted with a semiconductor chip, a display device, and a method for mounting a semiconductor chip.
The present application claims priority from Japanese Patent Application No. 2003-203478, the disclosure of which is incorporated herein by reference.
Electronic appliances, such as mobile phones or PDAs (Personal Digital Assistants or portable information terminals), have been required to have reduced sizes and weights and provide enhanced performance. This in turn requires their electronic components to be more densely incorporated upon the printed circuit board. In particular, thin flat-panel display devices, which are incorporated into such-an electronic appliance to provide as large display screen as possible, require their wired driver components accommodated around the screen to be packed as densely as possible. To meet this requirement, the chip-on-film (abbreviated as COF) technique has been often employed these days in which wirings of a flexible circuit board and output terminals of a semiconductor chip are directly bonded to each other to mount the semiconductor chip on the flexible circuit board.
With the COF technique, a pattern of the wirings on the flexible circuit board has to be formed corresponding to a pattern of the output terminals (bumps) of the semiconductor chip. The wiring pattern of the flexible circuit board may be often formed using a pattern forming technique referred to as a semi-additive or full-additive method as described in Japanese Patent Application Laid-Open No. 2000-286536.
Now, referring to FIGS. 1A through 1D, this conventional technique will be described below. As shown in FIG. 1A, a surface of a flexible insulating material base 100 is first covered with a seed layer 101 which is to serve as plated leads. Then, as shown in FIG. 1B, a patterned mask 102 of a photoresist material or the like is formed on the surface of the seed layer 101 to provide a desired pattern of wirings. Then, as shown in FIG. 1C, an electrically conductive material such as nickel or copper is deposited by electrolytic plating over exposed regions of the seed layer 101 to form a pattern of wirings 103. Additionally, as required, a surface conductive layer 104 of a dissimilar metal such as gold is formed on top of the patterned wirings 103 by electrolytic plating or a deposition method such as sputter or vapor deposition. Subsequently, as shown in FIG. 1D, the patterned mask 102 and the seed layer 101 underlying the mask 102 are removed, thereby forming a flexible circuit board with the desired pattern of wirings made up of seed layer portions 101A, the patterned wirings 103, and the surface conductive layer 104 on the insulating material base 100.
On the other hand, the array pattern of output terminals (bumps) of a semiconductor chip depends on the arrangement of terminals of an electronic appliance to be driven or the configuration of circuit blocks inside the semiconductor chip. In general, the array pattern may often be a non-uniform pattern of terminals, in the case of which the array includes bumps of different dimensions with the bumps of the same dimensions collected in groups, in which the groups are distributed unevenly.
To use the COF technique for mounting a semiconductor chip having bumps of different dimensions as mentioned above, it is necessary to form patterns of wirings of different widths corresponding to the dimensions of the bumps in order to connect the bumps to the wirings on the flexible circuit board with high accuracy. Forming such patterns of wirings is a critical design requirement for an electronic appliance whose performance is significantly affected by the magnitude of drive currents. In particular, this type of wiring pattern design is essential to the flexible circuit board which is incorporated into an organic EL display device, or a spontaneous light-emitting flat panel display receiving attention these days, in which the magnitude of drive currents have direct effects on display performance.
However, employing the conventional wiring pattern forming technique to form such patterns of wirings of different widths would lead to the following problem.
That is, forming patterns of wirings of different widths by electrolytic plating would cause a wiring material to be deposited in a larger thickness on a wider wiring but in a smaller thickness on a narrower wiring. This phenomenon results from the fact that a wider wiring provides a less voltage drop due to the resistance thereof than a narrower wiring during electrolytic plating. Upon bonding the bumps of the semiconductor chip to the patterned wirings of the flexible circuit board via an anisotropic conductive film by thermo-compression, such a difference in thickness between the patterns of wirings would readily cause unsuccessful bonding of neighboring wirings in a stepped portion.
This will now be explained more specifically in accordance with the example shown in FIG. 2. On a flexible circuit board 1, formed are a first wiring section 1A having a pattern of wider wirings 1a of the same size and a second wiring section 1B having a pattern of narrower wirings 1b of the same size. On the other hand, a semiconductor chip 2 is provided with a first bump section 2A having a pattern of wider bumps 2a of the same size and a second bump section 2B having a pattern of narrower bumps 2b of the same size. The wirings 1a and the bumps 2a having the same pattern and width as well as the wirings 1b and the bumps 2b having the same pattern and width are abutted against each other via an anisotropic conductive film 3, respectively, and then heated under a pressure P for thermo-compression.
At the boundary portion between the first wiring section 1A and the second wiring section 1B, the aforementioned difference in thickness between the wirings caused by the widths thereof defines a height difference between the levels of the bonding surfaces of the wirings (or stepped portion). Effecting a thermo-compression under this condition would cause the pressure to be insufficiently applied to a neighboring portion A around the stepped portion due to the existence of the stepped portion, resulting in an insufficient compression and thus unsuccessful bonding at the neighboring portion A.
To address this problem, the wirings 1a in the first wiring section 1A and the wirings 1b in the second wiring section 1B could be made the same in thickness. However, providing the same thickness to the wirings of different sizes would requires a special processing technique, which could be employed only with extreme difficulties to process such a fine pattern of wirings, thus causing an increase in the manufacturing costs of the flexible circuit board.